The project proposed here involves the study of terminal deoxynucleotidyl transferase, an unusual DNA polymerase which does not accept template instruction. The enzyme activity has been detected in normal thymus and bone marrow cells as well as in cells from certain types of human leukemia. Terminal transferase will be studied in normal bone marrow in order to identify a specific cell type which contains the enzyme activity. In addition terminal transferase will be studied in bone marrow cells which are differentiating.